Fix Broken Links in Documentation

[lscott-uw]

Documentation:

1.2 Installation
1.2.2.1 Fork and clone Graphitt
Contribute to Graphitti - Correct link location.

1.2.3
h5c++ link no longer works

2. Developer Documentation
   PlantUML Action
   Link to CloudBees github-action broken
   Introducing GitFlow
   Contributing link broken.

3. Testing
   3.1 Unit Tests
   docs/Testing/GoogleTest
   File no longer exists.

[lscott-uw]

Repo for CloudBees/plantuml-github-action no longer exists: https://github.com/cloudbees/plantuml-github-action

Need to verify if this action is still being utilized in the repo.

[lscott-uw]

Introducing GitFlow
Link already fixed in current shared development version, not yet pushed to master.

[lscott-uw]

GoogleTest
Check if this needs to be removed or redirected to a new page.